1. Field of the Invention
The invention relates to a wavelength converter for converting a first light signal of a first wavelength for transmitting data into a second light signal of a second wave-length, having signal input for receiving the first light signal, a signal output for emitting the second light signal, a modulatable optical semiconductor amplifier with an input, an output and modulation input, a first laser light source which with the semiconductor amplifier forms a first stage, with the input of the semiconductor amplifier being connected to the first laser light source and the modulation input of the semiconductor amplifier being connected to the signal input for cross amplification modulation and the first laser light source emitting light of a third wavelength, and wherein a second stage is connected to the output first stage by a coupling element, the second stage being provided with a second laser light source emitting light of the second wavelength and with a modulatable component.
Wavelength converters are required, inter alia, in optical data transmission systems in which data is transmitted by time or wavelength multiplexing.
In digital data transmission, data may be transmitted by light signals of two different levels, each corresponding to a binary signal. Since in data transmission, the error rate is influenced by the spacing between the two logic levels, it is desirable that the inter-level spacing of the input signal be not reduced by any wavelength conversion. In wavelength conversion the level spacing is determined by the so-called extinction factor, the extinction factor achievable with a single semiconductor amplifier being as a rule too low to maintain the level spacing of the input signal in the output signal. Cascading two semiconductor amplifiers for wavelength conversion will result in an increasing deterioration of the extinction, i.e., in a reduction of the extinction factor.
2. The Prior Art
In SIMON, J. C., et al.: Two-stages wavelength converter with improved extinction ratio, OFC 1995, San Diego, U.S.A., there is described a dual stage optical wavelength converter, by which a light signal may be converted from a first wavelength to a second wavelength.
The wavelength converter described in this prior publication is provided with two optical series-connected semiconductor amplifiers each of which amplifies a light signal applied to its input. A light signal from a laser light source having a wave-length of .lambda..sub.int and operating in a continuous wave is applied to the input of the first amplifier and does not, therefore, transmit any data. Furthermore, the two semiconductor amplifiers are each equipped with a modulation input connected to the signal input of the wavelength converter.
Hence, the light signal of wavelength .lambda..sub.in applied to the signal input of the wavelength converter modulates the amplification of the two semiconductor amplifiers, the amplification factors of which, because of saturation effects, will decrease as the level of the modulation signal increases.
The modulation signal of wavelength .lambda..sub.in and the light signal of wavelength .lambda..sub.int are coupled into the semiconductor amplifiers in opposite directions so that only the signal of wavelength .lambda..sub.int will leave the semiconductor amplifiers in the direction of the signal output.
If the level of the modulation signal of wavelength .lambda..sub.in is relatively high, the semiconductor amplifiers are driven to saturation so that the amplification factor and, hence, the levels of any output signals will be relatively low. If, by contrast, the level of the modulation signal of wavelength .lambda..sub.in in relatively low, then the amplification factor of the semiconductor amplifiers and the level of any output signals will be relatively high. That is to say that the polarity of the output signals of the two semiconductor amplifiers is inverted relative to the input signal of the wavelength converter.
Between the two semiconductor amplifiers there is arranged, on the one hand, a unidirectionally effective optical isolator and, on the other hand, an optical dampening member. The isolator blocks the light emitted by the second stage semiconductor amplifier in the direction of the first stage and this prevents disturbance of the first stage. The dampening member, on the other hand, sets the level of the output signal of the first semiconductor amplifier.
The described wavelength converter advantageously allows conversion of a first wavelength in a strictly optical manner into a second wavelength without intermediate electro-optic transducers.
Advantageously, the conversion is substantially immune from changes of the polarity and of the wavelength of the input signal, since amplification by the semiconductor amplifiers is substantially independent of polarization and wavelengths.
Nevertheless, the mentioned wavelength converter suffers from a variety of disadvantages.
Firstly, this wavelength converter inverts the signal polarity of the input signal. Thus, if several wavelength converters are arranged in a cascading manner, the number of cascaded wavelength converters must be considered in evaluating the signal. In an optical data transmission system this would be entail significant complexity, and it would limit flexibility.
Secondly, conversion by optical semiconductor amplifiers is dependent upon the position of the wavelengths of input and output signal. Thus, conversion by optical semiconductors works best when converting larger into smaller wavelengths.
Moreover, a distorted input signal is reproduced as a distorted signal at the output. This is a particular drawback where several wavelength converters are arranged in a cascade, as in such an arrangement noise is accumulated thus limiting the number of cascaded wavelength converters.
Moreover, the wavelength converter provides an output signal of a relatively unclean frequency spectrum ("chirp"). Because of the dispersion inherent in light waveguides, this, in transmitting the output signal through a light waveguide, will result in a distortion of the transmitted signal and, therefore, to a restriction of the distance over which transmission is possible without intermediate amplifiers or repeaters.
In U.S. Pat. No. 5,264,960, there is described a wavelength converter which is constructed of two stages, each stage being based upon the cross gain modulation (XGM--cross gain modulation) and provided with a modulatable optical semiconductor amplifier having an input, an output and a modulation input and a laser light source, the two stages being connected to each other by an optical coupling element. However, when cascading two XGM converters a chirped signal will result which is imprecise in terms of its wavelength. In the transmission of date over standard single-mode fibers this spectral uncleaness is, however, disadvantageous because it makes transmission of 10 Gbit/s impossible even over a small distance of 20 km and because signal distrotions are recognizable at low bit rates already (vide J. S. Perino; J. M. Wiesenfeld "Proceedings of 1994 Conference on Lasers and Electro-Optics and the International Electronics Conference CLEO/IQEC, Anaheim, Calif., USA, 8-13 May, pp. 298-299).